Create R 3.5.x docker files

[lcolladotor]

Ideally, we want everything to run on R 3.5.x. If we have docker files for R 3.5.x, then we won't need the wg_test flag anymore (mentioned in #10 and #11).

There are other docker related issues like #3 and #4. We'll leave this and those issues till later (I'll need to figure out the account details for updating the LIBD dockers that Jacobo created).

[lcolladotor]

Now this would be R 3.6. We might want to use the Bioconductor docker files http://bioconductor.org/help/docker/ For example https://github.com/LieberInstitute/RNAsp/blob/master/dockerfiles/r3.4.3_base.dockerfile#L62 won't work since https://bioconductor.org/packages/release/bioc/src/contrib/BiocGenerics_0.24.0.tar.gz doesn't work anymore